Structure-directing effect

Over the last few years, we have made a number of novel discoveries using reactive salt fluxes in the crystal growth experiment of mixed-metal oxides. The most important outcome that these salt-inclusion solids have demonstrated is the propensity for structure- directing effects of the employed salt. These hybrid solids have revealed fascinating solid-state structures ranging from nanoclusters to three-dimensional open frameworks of current interest. Solids featuring mag-... 

Specific structure-directing effects of some organic bases or cations When in the procedure BT Pr N is replaced by other organics, various pentasil-type zeolitic precursors are formed. It appears that specific zeolites are formed only when quaternary ammonium salts are used, their nature (structure) being essentially dependent on the length of the alkyl chains pure ZSM-8, ZSM-5 and ZSM-11 are obtained respectively with Et N+, Pr N+ and Bu N cations. TG data indicate that the latter fill nearly completely the zeolitic channel system (Table VIII). 

Burton, A., and Zones, S. (2007) Organic molecules in zeolite synthesis their preparation and structure-directing effects, in Introduction to Zeolite Science and Practice, 3rd Revised edn (eds ). Cejka, H. Van Bekkum, A. Cornia, and... 

Table 1 Structure directing effects by changing the Si02/Al203 ratios in the starting synthesis mixture (MFI / FER) with the starting composition of the reaction mixture of 28 Na20 / 6 B203 / z Al203 / 69 Si02 / 135 PA / 15428 H20 (z=0.95-3.20)...

However, the structure-directing effect of the template on the final framework structure was not elucidated even in this study. In situ studies into the structural features of the template species at the gel stage and during crystallization are needed to shed more light on this issue. 

The synthesis of pentasil zeolites is supported by organic cations. Argauer et al. (1) first described syntheses with tetraalkylammonium and tetraalkylphosphonium compounds. The organic cations may not only initiate and sustain a certain crystallization process but also may lead to products of a new structure. This "templating" behaviour is explained by the structure-directing effect of the organic cations in the process of crystallization (2,3). 

In consideration of the fact that even a template-free synthesis is possible it is not easy to understand that all different groups of compounds should have the same structure-directing effect. For a more plausible explanation we must consider the reaction mixture as a whole and discuss the combined effects of all components. Table III is an attempt to systematize the effects of different components. 

The +real structure directing effect is caused by the TEA, TPA, and TEA cations. These cations cause the structure directing effect also in the aluminium-free syntheses. 

Di-n-propylamine (DPA). Di-n-propylamine templates at least 7 structure-types in MeAPO synthesis. This multiplicity of possible structures makes the job of preparing one structure, free of competing structures more challenging. Fortunately, other synthesis factors can be used to focus the structure-directing effect of the template. Such factors as metal type, template... 

Conformational stability of an SDA reduces the disorder of the molecules during the formation of the framework. This may lead to stronger structure-directing effects [8, 9]. 

E Hadjoudis, J. Argyroglou, E. Lambi, and I. Moustakali-Mavridis, Structural-directing effects in the photochromism of anils, Mol Eng. 1, 67-74 (1991). 

The structure-directing effect means that the specific structure can be directed only by a specific organic species. For instance, crown ether 18-crown-6 in the synthesis of hexagonal faujasite (EMT) plays such a structure-directing role. Figure 2.18 shows a hypothetical model of an 18-crown-6 molecule in the supercage of EMT. [(Crown-, Na)+, OH ] is combined into the channel during the crystallization process.[34]... 

Further discussion of the templating or structure-directing effect of organic amines will be given in Chapter 5. 

Structure-directing Effect (SDE) and Templating in the Crystallization Process of Microporous Compounds... 

G. Sastre, J.A. Vidal-Moya, T. Blasco, J. Rius, J.F. Jorda, M.T. Navarro, F. Rey, and A. Corma, Preferential Focation of Ge Atoms in Polymorph C of Beta Zeolite (ITQ-17) and Their Structure-directing Effect A Computational, XRD, and NMR Spectroscopic Study. Angew. Chem., Int. Ed., 2002, 41, 4722-4726. 

Hadjoudis, E., Vitorakis, M., and Moustakali-Mavridis, I. (1986) Structural directing effects in solid state organic photochemistry, Mol. Cryst. Liq. Cryst., 137, 1-15. 

---